1. Field of the Invention
The present invention relates to a recording and reproducing apparatus which performs recording and reproducing with respect to an optical disc recording medium through laser light radiation, and more particularly to recording and reproducing apparatus having an adjustment unit which compensates for changes in characteristics of the laser light caused by changes in temperature, such as spherical aberration, focus bias, and tilt. Also, the present invention relates to a method of calculating a temperature characteristic compensation operation coefficient for an adjustment value of the adjustment unit. Further, the present invention relates to a reproducing apparatus having the adjustment unit as well as performing reproducing of the optical disc recording medium.
2. Description of the Related Art
As technology for recording and reproducing digital data, there is a data recording technology which uses an optical disc recording medium (hereinafter simply referred to as “optical disc”), such as a CD (Compact Disc), MD (Mini-Disc), and DVD (Digital Versatile Disc). An optical disc is a general name of a recording medium in which laser light is radiated on a disc composed of a metal thin film that is protected by plastic and a signal is read in accordance with a change in the reflected light from the disc.
Optical discs are divided into a reproducing dedicated-type, known as, for example, CD-ROM and DVD-ROM, and a user data recordable-type which is known as MD, CD-R, CD-RW, DVD-R, DVD-RW, DVD+RW, and DVD-RAM. On the recordable type optical disc, data can be recorded using a magneto-optical recording method, a phase-change recording method, or a pigment-film-change recording method. A pigment-film-change recording type optical disc is a so-called a write-once recording type optical disc. Since data can be recorded on the pigment-film-change recording type optical disc only once and no data rewriting is possible, the pigment-film-change recording type optical disc is appropriate to data preservation. On the other hand, according to a magneto-optical recording type optical disc or a phase-change recording-type optical disc, it is possible to rewrite data thereon, and thus they are used for various uses including recording of various kinds of content data such as music, videos, games, and application programs.
Further, a high-density optical disc, which is called a Blu-Ray disc (registered trademark, hereinafter also referred to as “BD”) has recently been spread, and remarkably high capacity of an optical disc has been sought.
A high-density disc such as BD has a disc structure in which a cover layer of about 0.1 mm is provided in the disc thickness direction, and data is played from (or recorded on) the disc through a combination of a so-called blue-violet laser having a wavelength of about 405 nm and an object lens of which the NA (Numerical Aperture) is about 0.85.
However, as is known to all, in a recording and reproducing apparatus that performs recording and reproducing of an optical disc, a focus servo operation for controlling the focusing position of a laser light onto a recording surface of the disc and a tracking servo operation for controlling the laser light to trace a track (formed by a pit string or a groove) on the disc are performed.
Regarding the focus servo, it is known that applying an appropriate focus bias to a focus loop is necessary for an appropriate servo operation.
Particularly, in the case of a high-density disc, it is necessary to perform correction of spherical aberration to cope with thickness errors in the cover layer or a recording layer having a multilayer structure, and thus a recording and reproducing apparatus having a spherical aberration correction tool using an expander or a liquid crystal device, which is provided inside an optical pickup, has been developed (for example, see Japanese Unexamined Patent Application Publication Nos. 2002-352449 and 10-269611).
In a recording and reproducing apparatus provided with a lens having a high NA, such as BD, the margin of the focus bias or the spherical aberration are narrow, and thus their adjustment is necessary.
A focus bias adjustment method has been known, for example, as in Japanese Unexamined Patent Application Publication No. 2000-285484.
Also, a spherical aberration adjustment method has been known, for example, as in Japanese Unexamined Patent Application Publication No. 9-251645.
Also, an optical disc recording and reproducing apparatus that performs tilt correction to suppress a coma aberration has been known. As examples of tilt correction methods, a method of adjusting the slope of an optical disc, a method of performing an aberration correction though a liquid crystal device inserted onto an optical path have been known.
Here, according to a method to adjust the above-described optical values such as the values of focus bias, spherical aberration, and tile, evaluation values for the respective signals recorded under the condition that the respective adjustment values are set are obtained by reading the signals while changing the adjustment values within a specified range, and the optimum adjustment value is determined on the basis of the result of obtaining the evaluation values.
As described above, by obtaining the optimum adjustment value on the basis of the evaluation values actually measured, the differences by objects of the optical pickup or the optical disc, and further the consecutive changes of the characteristics can be absorbed.
The adjustment operation of the various kinds of adjustment values based on the above-described evaluation values actually measured is performed according to timing in which the optical disc is loaded, for example, as illustrated in FIG. 18.
The recording and reproducing operation is executed after the adjustment values such as the values of focus bias, spherical aberration, and tilt are adjusted to optimum values by the corresponding startup operation.
Also, before the start of the startup operation, the setting of the initial adjustment value (i.e. the setting of the startup initial adjustment value) for performing the corresponding startup operation is performed, which will be described later.
However, it is known that the optimum point of the adjustment values such as the values of focus bias, spherical aberration, and tilt changes depending upon the temperature change. That is, the occurrence of a temperature change causes a characteristic change in optical components such as an object lens, and the occurrence of the characteristic change causes the change of the optimum point. Also, a change in the optimum point also occurs due to a wavelength change in the laser light according to the temperature change.
The adjustment value of which the optimum point is changed depending upon the temperature is called “a characteristic change temperature-dependent adjustment value”.
FIG. 19 is a diagram illustrating the change characteristic of the optimum point for the temperature change of the characteristic change temperature-dependent adjustment value.
In this case, in FIG. 19, as an example of the characteristic change temperature-dependent adjustment value, a spherical aberration correction value (hereinafter also referred to as “SA”) is exemplified.
In FIG. 19, “characteristics of object A during shipment” that is indicated by a black solid line indicates the change characteristic of the optimum SA for the temperature change of the optical pickup as the object A during shipment, and “the characteristic of object B during shipment” that is indicated by a black dashed line indicates the temperature-optimum SA characteristic of the optical pickup as the object B during shipment.
Also, “average characteristic during shipment” that is indicated by a black dashed dot line indicates the average temperature-optimum SA characteristic of a plurality of optical pickups.
Also, “characteristics of the consecutively changed object A during shipment” that is indicated by a gray solid line indicates the temperature-optimum SA characteristic of object A after the lapse of a predetermined time from shipment, and “the characteristic of the consecutively changed object B” that is indicated by a gray dashed line indicates the temperature-optimum SA characteristic of object B after the lapse of a predetermined time from shipment in the same manner.
As illustrated in FIG. 19, the change characteristic of the optimum SA for the temperature change (i.e. the temperature-optimum SA characteristic) may approximate a straight line.
Also, as shown in the drawing, it is typical that the temperature-optimum SA characteristic differs for each object of the optical pickup, and even in the case of the same object, the consecutive change occurs.
Also, it is typical that during shipment, the characteristics of the respective objects (in the drawing, object A and object B) differ from the average characteristic (i.e. both the slope and offset of the straight line) after the consecutive changes. That is, the difference between the objects becomes relatively large.
Also, although not illustrated in FIG. 19, the temperature-optimum SA characteristic is changed by objects of the optical disc.